Big fish, small fish

Could someone please explain once more why you have to do unto others that which you want to have done unto you? This reminds me of a story:

Two fishes to feed 5000 CC Grauesel

Abu Hassan al-Mutakallim al-Hikma, utmost authority in the study of Divine Knowledge and Ethics, hearing too much and too often about the aforementioned Mullah Nasreddin, came to examine this man in his village of Horto. In order to make the small Mullah feel at ease and in confidence, open to investigation, Abu Hassan invited him to eat together at the local inn. That day they served fish.

When the ordered course arrived, at last, it was easy to observe that on the platter there where two fishes; one larger and the second much smaller, both attractively prepared and smelling definitely appetising, sprinkled with fresh parsley. Nasreddin reached out without hesitation and pulled the large fish on his own plate. The distinguished guest and moralist looked at him in consternation and couldn’t contain his righteous observation.

“My dear colleague, isn’t this act selfish and immoral?”

“Why, Maulana, what would you do in my stead?” replied Hodja, looking up from his steaming dish with candid eyes.

“I would have, of course,” said the distinguished scholar, “taken the smaller fish for myself and given the larger one, unselfishly, to my fellow man.”

“And here we are, doing the right thing!” concluded Nasreddin elegantly tossing the small fish into the guest’s plate.